What is Sora's Kyuubi power compared to Naruto's?

Sora has the Kyuubi's leftover chakra from when it attacked the leaf village. Naruto has the actual Kyuubi inside his body. What fraction of the Kyuubi's power does Sora posses and how much stronger does having the actual beast make Naruto? I saw that Sora was able to achieve 4 tails but theres no way he'd be able to achieve more than that could he?